1. Field of the Invention
Embodiments of the present invention relate generally to disk drives and, more particularly, to a method of measuring non-coherent runout for such drives.
2. Description of the Related Art
A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer, which includes a read element and a write element, that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo system is generally implemented. The servo system uses servo data read from a “servo wedge” on the disk surface to align the transducer with the desired track, where the servo data may include the track number as well as “servo bursts” that indicate how far the recording head is from the ideal track center line. The servo data are previously written on the disk surface by the drive itself using a self-servo-writing procedure or by an external device, such as a servo track writer (STW). In either case, an additional factory calibration for each track present on the disk drive may be required to compensate for small errors in the position of the servo bursts written on the disk surface. Because modern disk drives typically include hundreds of thousands of tracks, such factory calibration is a time-consuming part of the manufacturing process.
In an ideal disk drive, the tracks of a disk are non-perturbed circles situated about the center of the disk. As such, each ideal track includes a track centerline that is located at a known constant radius from the disk center. In practice, however, writing non-perturbed circular tracks to a disk is problematic due to imperfections in the media itself and/or in the position control of the device writing the servo bursts caused by mechanical effects, e.g., vibration, bearing defects, inaccuracies in the STW, disk clamp slippage, etc. Thus, the servo bursts that define each track are generally written with an offset from the ideal non-perturbed circular track shape. Positioning errors created by the offset between the real servo burst locations and the ideal track location are known as repeatable runout (RRO).
RRO can be divided into two components: coherent and non-coherent. The coherent portion of RRO is the portion that is approximately the same in a group of adjacent tracks and changes slowly across the disk surface. Coherent RRO is typically caused by disk eccentricity, clamping distortions, and other factors that alter the shape of relatively large portions of the disk, thereby producing irregularities in the placement of servo bursts from ideal circular tracks that are substantially similar for adjacent tracks. Coherent RRO can be large with respect to track width, for example on the order of multiple track widths. The non-coherent portion of RRO is generally much smaller than the coherent portion, is different for each track, and is due to imperfections in the media magnetic layer or in the position control of the transducer while writing the servo bursts.
Without additional correction to the servo bursts as written to a disk, the non-ideal shape of the tracks as defined by the servo bursts creates two problems. First, the transducer positioning function is made more complicated during read and write operations because the servo system needs to continuously reposition the transducer during track following to keep up with the rapidly changing radius of the track centerline as defined by the non-coherent RRO of the servo bursts, rather than following the more smoothly changing radius of the coherent RRO perturbations. Second, the perturbed shape of these tracks due to non-coherent RRO can result in problems such as track misregistration errors during read and write operations and “track squeeze,” i.e., adjacent tracks that are spaced too close together.
Disk drive manufacturers have developed different techniques to compensate for coherent and non-coherent RRO. Typically, coherent RRO is compensated by injecting appropriate signals into the disk drives actuator that results in the head accurately following the coherent RRO. On the other hand, non-coherent RRO is often compensated by adding correction factors (sometimes called wedge offset compensation values) to the measured head position such that the head does not follow the non-coherent RRO. In one typical embodiment these wedge offset compensation values are computed during the post-manufacturing test process and written on the disk surface. Because it is typically undesirable for these techniques to correct for the coherent portion of RRO, disk drive manufacturers have developed techniques to separate the coherent and non-coherent RRO for each track of a disk drive. However, techniques known in the art for separating coherent and non-coherent RRO require substantial computation time and/or memory. For example, prior to calculating compensation values for non-coherent RRO, coherent RRO can be estimated for a plurality of zones on a disk surface by measuring the average RRO for multiple tracks in each zone, which can be a time-consuming procedure. Alternatively, coherent RRO can be determined while computing non-coherent RRO for a given track on the fly by storing the total RRO of recently measured tracks, e.g., the previous 100 tracks, computing coherent RRO based on the average RRO of these tracks, and defining non-coherent RRO as the total RRO minus the average RRO. Such a procedure requires significant additional computational resources and data storage to calculate the compensation values for non-coherent RRO.
In light of the above, there is a need in the art for an improved method of measuring non-coherent RRO for a disk drive when calculating wedge offset compensation values.